Gui: Unavailable toolbars default to visible when made available

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FreeCAD#9208 (comment)

Issue:
- Unavailable toolbars default to not visible (as they are unavailable)
- However, when made available, they remain not visible. There may be
  cases when this is indeed the wanted behaviour, but they are not the
  ones we are facing.
- In the latter, the user expects the toolbars to show (and the
  configuration file to be saved with all toolbars showing.

Solution:
- Change the default behaviour of Unavailable toolbars when forcing
  them to be available, so that they are visible when made available.
- If then the case arises that we have toolbars where we would prefer
  then to be left not visible when making them available (so they are
  listed in the toolbar context menu to make them visible, but not
  visible per default), then we would need to create another toolbar
  policy class "UnavailableHidden".
- I chose not to add it directly (although it is straightforward) in
  fear that we will never need it, and it would make the code more
  complex to understand and thus maintain.

src/Gui/ToolBarManager.cpp

@@ -536,7 +536,16 @@ void ToolBarManager::setState(const QString& name, State state)
 
             tb->toggleViewAction()->setVisible(true);
 
-            showhide(tb, policy);
+            // Unavailable policy defaults to a Visible toolbars when made available
+            auto show = visibility( policy == ToolBarItem::DefaultVisibility::Visible ||
+                                    policy == ToolBarItem::DefaultVisibility::Unavailable);
+
+            if(show) {
+                tb->show();
+            }
+            else {
+                tb->hide();
+            }
         }
         else if (state == State::ForceHidden) {
             tb->toggleViewAction()->setVisible(false); // not visible in context menus

src/Gui/ToolBarManager.h

@@ -44,7 +44,8 @@ class GuiExport ToolBarItem
         Hidden,      // toolbar hidden by default, visibility toggle action is enabled
         Unavailable, // toolbar visibility is managed independently by client code and defaults to
                      // hidden, visibility toggle action is disabled by default (it is unavailable
-                     // to the UI).
+                     // to the UI). Upon being forced to be available, these toolbars default to
+                     // visible.
     };
 
     ToolBarItem();